In most trilobites, the posterior thoracic region or abdomen terminates in a shield called pygidium. This may result from the fusion (or lack of separation during growth) of a number of tergites. Extreme variation in size occur for this portion of the carapace, and the extension of the pygidium seems related to the number of thoracic segments. In the example in figure 2, the pygidium is small-the trilobite is called micropygous-but the number of thoracic segments is relatively large (twenty for...

The first edition of my atlas contained a firsthand description of my encounter with the Middle Cambrian Paradoxides beds of Eastern Newfoundland. This field trip was singled out as deserving a special mention in my book, as it represented a significant episode of scientific discovery. It also told an exciting story linking together Carl von Linne's first identified trilobite, Entomolithus paradoxus, the Newfoundland trilobites that I dug out, and spectacular evidence of continental drift. The...

Two views of Philonyx philonyx Richter and Richter, Hamar Laghdad Formation, L. Devonian, Alnif, Morocco (xl.9). These plates offer another glimpse at the extravagant and delicate trilobite structures that have been uncovered recently by the expert use of airabrasive techniques. For Pbilonix also, a formal description of the complete morphology is still lacking. As seen in this example, the anterior border of the cranidium projects forward in an upturned,...

Among the remains of early life on earth, the fossil record we find buried in ancient sedimentary rocks bears evidence of an extraordinary group of marine creature, the trilobites. The position of these invertebrates in the evolution of the animal kingdom is extraordinary because of their early ascent to a high level of functional complexity, described in fascinating detail by their persistent and ubiquitous fossil remains. Trilobites could see their immediate environment with amazingly...

The atlas is organized in six main sections corresponding to the geologic periods that contain the fossil record of trilobites, from their appearance at the beginning of the Cambrian to their demise at the end of the Permian. Each section is preceded by a listing of the trilobite families that are found in that particular geologic time interval, in order of appearance, following the information available in the Treatise (Moore 1959). The genuses illustrated by atlas plates are indicated in...

Atlas of invertebrate macrofossils. New York John Wiley &amp Sons. An extensive collection of small but good quality black and white photographs, with good coverage of trilobites. Pinna, G. 1972. The dawn of life. New York World Publishing. Invertebrate fossils in captivating color photographs. Ransom, J. E. 1964. Fossils in America. New York Harper and Row. A useful introduction and field guide to fossil collecting. Shrimer, H. W. and R. R. Shrock, 1944. Index fossils...

I wish to express my gratitude to a great number of persons who over the years have contributed encouragement, inspiration, and practical help. I am particularly indebted to the late Dr. Eugene Richardson and to Matthew Nitecki of the Field Museum of Natural History, Chicago, for making the museum collections available for the selection of specimens to be used in this work, for the loan of those specimens, and for many helpful discussions. A great debt of gratitude is due to Dr. Jan Bergstrom...

The head shield or cephalon resulted from the fusion of a number of tergites (five or seven) and often carries telltale memory of the original segmentation. It is the most significant and characteristic part of the trilobite morphology. The outline of the cephalon may be semicircular to ogival in its anterior portion, straight or gently curved at the posterior margin, where articulation with the first thoracic segment occurs. The lateral and anterior margin of the cephalon are inflected into...

Zacanthoides typicalis Walcott xll . Chisholm Shale, M. Cambrian, Half Moon Mine, Pioche, Nevada. Specimen collected by Afton Fawcett. RLS coll., id., now at FMNH. Once again it is worth showing the comparison berween standard photography of the specimen in air a and what can be seen with xylene immersion b . The long axial spine originates from the eigth thoracic axial ring. Walcott xl5.8 . Same origin as for preceding plate. Specimen collected by Afton Fawcett. RLS coll., id.,...

The two parts of the specimen in plate 49 are now assembled to show how the trilobite was originally found. In no way could the trilobite bring the cephalic margin to match the border of the long pygidium. Even so, enrollment protection, also in view of the presence of a robust telson, not preserved here, protruding from the tip of the pygidium. A perfectly enrolled specimen of the Ordovici an trilobite Foerste from Waynesville, Ohio x9 . RLS coll. now at FMNH. Probable life posture of the...

Composition of the animal kingdom through geologic time. Only the ptincipal phyla are indicated names in a box , along with some of the classes they encompass vertical lines . Related subclasses are connected by an empty box. The time scale indicated was adopted by the Geological Society of London in 1964 from Kummel 1970 . The histogram at the bottom of the figure indicates the percentages of living species, excluding insects partially from Easton 1960 Shrock and Twenhofel 1953 and from a...

The right eye of Phacops latifrons Bronn , from the Devonian of Germany. Loaned by E. N. K. Clarkson. xl4 Left eye of Phacops rana crassituberculata Stumm, from the Devonian Silica Shale at Sylvania, Ohio xl3 . RLS coll. now at FMNH. Specimen whitened with magnesium oxide. The lenses at the top of the visual surface appear incompletely developed. Right eye of another specimen of Phacops rana crassituberculata Stumm x 3 . RLS coll. now at FMNH. Specimen whitened with magnesium oxide. The lenses...

For a number of trilobite species, the various stages of growth ontogeny from the larval to the adult form are known with great detail from the fossil record. Three major periods of growth are recognized. The protaspidperiod extends from the hatching of the egg to the first appearance on the single-piece dorsal shield of a transverse suture, defining the cephalon and the so-called transitory pygidium. During this period, a larval ridge may be the precursor of the axial lobe. The size of the...

3.1 Introduction to Trilobite Morphology The general description of the distinctive features of the Arthropoda given in chapter 2 applies to the Trilobita as well. Here, however, the picture is less complete than for the living arthropods, since, of course, our evidence must be based only on the fossil record. Several details of the anatomy and physiology of trilobites are inferred by analogy with what is known about the character of arthropods in general. As much as this analogy may be useful,...

A schematic view of the range of optical structures in this type of eye is shown in figure 8, adapted from Lindstrom 1901 . In addition to the basic hexagonal prism design, other lens contours were present, including square prisms. The optical behavior of these elements, while seemingly obvious in the simple biconvex lenses of Sphaerophthalmus, conceals a surprising feat, which will become apparent in the case of the long prisms of Asaphus and Illaenus in particular. Such prisms were made of...

In all growth stages of Olenellus see plates 52-60 . In the early stages of growth as in the examples shown in this plate, the eye lobe extends over a larger portion of the cranidial length than in the fully grown individuals such as that of plate 1. Other late meraspid exuviae are shown in b xl.9 in an assemblage of one internal mold and two external impressions of approximately equal size. This and other similar assemblages from the same layers suggest some kind of size sorting in molting. P....

Very little can be added to the description of the physical principles involved and the technique of fossil photography given by Franco Rasetti in the Handbook of Paleontological Techniques Kummel and Raup 1965 . As a physicist, I have appreciated Rasetti's suggestions tremendously and have endeavored to translate them into practice as much as possible. Although it is not generally recommended for high resolution work, 35mm. film was used for all of the photographs. The use of slow,...

Computer-generated reconstruction of the distribution of land masses in the Middle Cambrian. Oblique polar view, showing the component parts of Gondwana and Baltica that became Maritime North America and Europe after the assembly and subsequent breakup of the supercontinent Pangea. These components are shown with present-day coastline contours, to facilitate recognition. The actual contours of the Middle Cambrian coastlines may have differed in details from the present configuration. This...

Dalmanitina socialis Barrande x l. 3 . Letna beds, U. - M. Ordo vician, Vesela, Bohemia. Loaned by FMNH. Assemblage of trilobite parts, remarkably well preserved in quartzite. In addition to Dalmanitina socialis, two cephala of Cryptolithus tesselatus are clearly visible. Due to the high resolution of this photograph, surface details of the carapace as well as the structure of several schizochroal eyes can be identified with the aid of a magnifier. Barrande , U.-M....

In modern arthropods the structural unit ommatidium is made of a sequence of functional subunits see fig. 7 . Facing the outside world is the dioptric apparatus consisting of a corneal lens in optical contact with a crystalline cone. Tiny images from a narrow field of view appear at the tips of the cones. Proceeding toward the interior of the eye beyond the cone there are two types of sttuctures. In the so-called apposition eyes mostly found in diurnal insects, Crustacea, etc. , the...

The biconvex lenses of phacopid trilobites are made up of doublet structures that were constructed for an unmistakable purpose to correct for the large spherical distortion aberration of simple thick lenses. I reached this rather surprising conclusion after examining the large amount of evidence on lens morphology that had been accumulated by Euan N. K. Clarkson 1968 during his systematic investigation of trilobite eyes. A preliminary account of this discovery was contained in the first edition...